1. Field of the Disclosure
The present disclosure relates to a wireless sensor apparatus for detecting a motion or the like of an object on the basis of a reflected wave of the object by using a wireless wave.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2004-245602 discloses an apparatus configured to feed an output signal from an oscillator to an antenna for radiating a wireless wave, receive a reflected wave obtained while this wireless wave is reflected by an object, and detect an orientation and a motion of the object on the basis of the received reflected wave.
FIG. 3 shows a wireless sensor apparatus configured to detect a motion or the like of an object by using a pulse signal output from an oscillator. A wireless sensor apparatus 1 shown in FIG. 3 is provided with two transmission and reception antennas A1 and A2. Two pulse signals are output at a same timing from an RF oscillating circuit 2 to be fed to the transmission and reception antennas A1 and A2 and also sent to mixer circuits 3a and 3b as local signals. Wireless waves are radiated from the transmission and reception antennas A1 and A2 which are supplied with the pulse signals, and reflected waves which are reflected from an object O are respectively received by the transmission and reception antennas A1 and A2. The received reflected waves are input to the mixer circuits 3a and 3b as electric reception signals and mixed with a part of the pulse signals input from the RF oscillating circuit 2 while being temporally overlapped. Signals output from the mixer circuits 3a and 3b are input to a differential amplification circuit 4 for differential amplification, and low frequency components (including a DC signal) are taken out in a low-pass filter (LPF) 5. The signals passing through the LPF 5 are converted into digital signals by an A/D converter which is not shown in the drawing and taken in into a signal processing circuit 6.
In the signal processing circuit 6, the presence or absence of the motion of the object O is detected on the basis of the signal of the low frequency components output from the low-pass filter (LPF) 5. If the object in a halt state, as outputs of mixer circuits 3a and 3b on left and right have no changes, an output of the differential amplification circuit 4 also has no change. However, if the object O moves, the outputs of mixer circuits 3a and 3b on left and right become different from each other, and a differential amplification is conducted in the differential amplification circuit 4. Thus, the motion of the object O can be detected on the basis of a change of the output signals. A differential amplification output of the differential amplification circuit 4 appearing along with the motion of the object O is a low frequency component, and this output is subjected to the low-pass filter 5 to remove high frequency components and unnecessary noise for only taking out a necessary signal.
However, in the wireless sensor apparatus 1 shown in FIG. 3, as the transmission and reception antennas A1 and A2 are supplied with the pulse signals at the same time from the RF oscillating circuit 2, a problem occurs in which the wireless waves radiated from the respective antennas A1 and A2 at the same time function as mutual disturbing waves for causing an interference. As in the wireless sensor apparatus 1 described above, in a case where the wireless waves are radiated from the two transmission antennas A1 and A2 at the same time in order to improve an antenna reception sensitivity, it is necessary to secure an isolation between the antennas which significantly affects the antenna reception sensitivity and a dynamic range. However, in a case where a distance between the antennas is increased to cancel the interference, the size of the apparatus becomes larger, and therefore a problem occurs in which it is difficult to satisfy both the antenna reception sensitivity and the miniaturization of the apparatus.
These and other drawbacks exist.